1. Field of the Invention
This invention relates to silicone rubber compositions with improved flame retardancy and to silicone rubber compositions having transparency and flame retardancy after curing.
2. Description of the Prior Art
Although silicone elastomers have various excellent characteristics, there is the drawback that they are combustible. Therefore, many methods for imparting flame retardancy have been proposed. For example, a silicone rubber compound with a platinum-containing material is known from U.S. Pat. No. 3,514,424. However, the use of platinum-containing material alone does not impart satisfactory flame retardancy. Many studies have attempted to improve the self-extinguishing ability by using other compounds in combination with the platinum-containing material. For example, the following compositions have been proposed: silicone rubber compounded with a platinum compound and fumed titanium dioxide as shown by U.S. Pat. No. 3,635,874, silicone rubber compounded with a platinum compound and a finely pulverized manganese carbonate as shown by U.S. Pat. No. 3,936,476, silicone rubber compounded with a platinum compound and (FeO).sub.x (Fe.sub.2 O.sub.3).sub.y where the ratio of x to y is 0.05 to 1.0 as shown by Japanese Pat. No. Sho 51[1976]-35501 and silicone rubber compounded with a platinum compound and .gamma.-Fe.sub.2 O.sub.3 as shown by Japanese Pat. No. Sho 53[1978]-44501.
Although these known compounds exhibit excellent flame retarding effects to a certain degree, they have the drawback that transparent silicone rubber cannot be obtained. On the other hand, there is an increasing demand for flame retardant silicone rubber having transparency through which a wiring section can be seen, from the standpoint of prevention of accidents especially in the fields of electrical and electronic devices.
In the case of a composition containing the above-mentioned platinum compound and fumed titanium dioxide, the electrical characteristics of the silicone elastomers are impaired. In the case of a composition containing the above-mentioned platinum compound and manganese carbonate, vulcanization is impaired using an acyl organic peroxide as a vulcanizing agent. In the case of a composition containing the above-mentioned platinum compound and (FeO).sub.x (Fe.sub.2 O.sub.3).sub.y, the effect on the self-extinguishing ability is weak unless (FeO).sub.x (Fe.sub.2 O.sub.3).sub.y is compounded in a large quantity. On the other hand, the mechanical characteristics of silicone elastomers are impaired by adding a large amount of (FeO).sub.x (Fe.sub.2 O.sub.3).sub.y.